The present invention pertains to a bandpass filter and more specifically to a voltage tunable combline filter.
Bandpass filters are filters which selectively pass signals having a certain frequency. Signals outside this frequency bandwidth are rejected so that they do not interfere with the desired signal.
In many applications, a variable range of signal frequencies is desired. The prior art meets this demand in a variety of ways. One such, is by using switchable fixed filters. This is a number of bandpass filters each tuned to a fixed frequency bandwidth. In order to pass different signals over a variety of frequencies, a number of fixed bandpass filters are used. The signals can then be switched between the filters to obtain the signal with the desired frequency.
The alternative to using a large number of filters is to produce a tunable bandpass filter. Some of these filters use tuning diodes for tuning. While this will allow a filter to selectively pass a wider range of frequencies, the components used in biasing the tuning diodes cause problems. They act as parasitic elements causing signal loss and stray capacitance which causes detuning of the filter. The biasing of filters using tuning diodes will be described in greater detail later in the application.